An ion source is a device that causes gas molecules to be ionized and then accelerates and emits the ionized gas molecules and/or atoms toward a substrate. Such an ion source may be used for various purposes, including but not limited to cleaning a substrate, surface activation, polishing, etching, and/or deposition of thin film coatings/layer(s). Example ion sources are disclosed, for example, in U.S. Pat. Nos. 6,359,388; 6,037,717; 6,002,208; 5,656,819, 6,815,690, Ser. Nos. 10/986,456, and 10/419,990, the disclosures of which are all hereby incorporated herein by reference.
FIGS. 1-3 illustrate a conventional Hall-effect, cold cathode, closed-drift type ion source. In particular, FIG. 1 is a side cross-sectional view of an ion beam source with an ion beam emitting slit defined in the cathode, FIG. 2 is a corresponding sectional plan view along section line II-II of FIG. 1, and FIG. 3 is a corresponding sectional plan view along section line III-III of FIG. 1. As can be seen in FIGS. 2-3, the ion source may have an oval and/or racetrack-shaped ion beam emitting slit although other types of slits such as a circular slit may instead be used. Other suitable shapes may also be used.
Referring to FIGS. 1-3, the ion source includes a hollow housing made of a highly magnetoconductive (or permeable) material such as iron, which is used as a cathode 5. The cathode 5 includes each of an inner cathode 5a and a one-piece outer cathode 5b. The outer cathode 5b may include cylindrical or oval side wall 7 and a closed or partially closed bottom wall 9; whereas the inner cathode 5a includes an approximately flat top wall 11 in which a circular or oval ion emitting slit and/or aperture 15 is defined. The slit 15 is defined at least partially between the inner cathode 5a and the one-piece outer cathode 5b. The bottom 9 and side wall(s) 7 of the cathode are optional. Ion emitting slit/aperture 15 includes an inner periphery as well as an outer periphery.
Deposition and/or plasma maintenance gas supply aperture or hole(s) 21 is/are formed in bottom wall 9. The flat top wall of the cathode functions as an accelerating electrode. A magnetic system including a cylindrical permanent magnet(s) 23 with poles N and S of opposite polarity is placed inside the housing between bottom wall 9 and top wall 11. The purpose of the magnetic system with a closed magnetic circuit formed by the magnet 23 and cathode 5 is to induce a substantially transverse magnetic field (MF) in an area proximate ion emitting slit 15. The ion source may be entirely or partially within a wall 50. In certain instances, wall 50 may entirely surround the source and substrate 45, while in other instances the wall 50 may only partially surround the ion source and/or substrate.
A circular or oval shaped conductive anode 25, electrically connected to the positive pole of electric power source 29, is arranged so as to at least partially surround magnet 23 and be approximately concentric therewith. Anode 25 may be fixed inside the housing by way of insulative ring 31 (e.g., of ceramic). Anode 25 defines a central opening therein in which magnet 23 is located. The negative pole of electric power source 29 is connected to cathode 5, so that the cathode is negative with respect to the anode (e.g., the cathode may be grounded in certain example non-limiting instances).
Generally speaking, the anode 25 may be biased positive by several hundred to a few thousand volts. Meanwhile, the cathode (inner and/or outer portions thereof) may be held at, or close to, ground potential. This is the during ion source operation.
The conventional ion beam source of FIGS. 1-3 is intended for the formation of a unilaterally directed tubular (in the case of a standard beam collimated mode for example) ion beam, flowing in the direction toward substrate 45. Substrate 45 may or may not be biased in different instances. The ion beam emitted from the area of slit/aperture 15 is in the form of an oval (e.g., race-track) in the FIG. 1-3 embodiment, although other shapes may be used.
The conventional ion beam source of FIGS. 1-3 can operate as follows in a depositing mode when it is desired to ion beam deposit a layer(s) on substrate 45. A vacuum chamber in which the substrate 45 and slit/aperture 15 are located is evacuated to a pressure less than atmospheric, and a depositing gas (e.g., a hydrocarbon gas such as acetylene, or the like) is fed into the interior of the source via gas aperture(s) 21 or in any other suitable manner. It is possible that the depositing gas may instead be introduced into the area between the slit 15 and substrate 45. A maintenance gas (e.g., argon) may also be fed into the source in certain instances, along with the depositing gas. Power supply 29 is activated and an electric field is generated between anode 25 and cathode 5 (including inner 5a and outer 5b), which accelerates electrons to high energy. Anode 25 is positively biased by several hundred to a few thousand volts, and cathodes 5a and 5b are at ground potential or proximate thereto as shown in FIG. 1. Electron collisions with the gas in or proximate aperture/slit 15 leads to ionization and plasma is generated. “Plasma” herein means a cloud of gas including ions of a material to be accelerated toward substrate 45. The plasma expands and fills (or at least partially fills) a region including slit/aperture 15. An electric field is produced in slit 15, oriented in the direction substantially perpendicular to the transverse magnetic field, which causes the ions to propagate toward substrate 45. Electrons in the ion acceleration space in and/or proximate slit/aperture 15 are propelled by the known E×B drift (Hall current) in a closed loop path within the region of crossed electric and magnetic field lines proximate slit/aperture 15. These circulating electrons contribute to ionization of the gas (the term “gas” as used herein means at least one gas), so that the zone of ionizing collisions extends beyond the electrical gap between the anode and cathode and includes the region proximate slit/aperture 15 on one and/or both sides of the cathode.
For purposes of example, consider the situation where a silane and/or acetylene (C2H2) depositing gas is/are utilized by the ion source of FIGS. 1-3 in a depositing mode. The silane and/or acetylene depositing gas passes through the gap between anode 25 and the cathodes 5a, 5b. 
Unfortunately, ion sources suffer from the problem that during use the electrode(s) (e.g., cathode and/or anode) erode over time. For example, consider a situation where the cathode (or anode) is made of steel—which includes iron. During use of the ion source, exposed surface portions of at least the cathode are prone to erosion. This type of electrode erosion is problematic for a number of reasons. First, significant erosion of the cathode over time can cause the width of the slit (i.e., the magnetic gap) to significantly change which in turn can adversely affect ion beam processing conditions and lead to non-uniform coatings, etchings, etc. When enough erosion has occurred to cause the width of the slit/gap to sufficiently change, the electrode(s) have to be replaced with entire new electrode(s).
In view of the above, it will be appreciated that there exists a need in the art for an ion source (and/or corresponding method) that is capable of efficiently dealing with the issue of electrode erosion.